This application claims the priority of German Patent Application Serial No. 200 14 361.1, filed Aug. 19, 2000, the subject matter of which is incorporated herein by reference.
The invention relates to a B-column, as component of a body for a motor vehicle, comprising a longitudinal steel profile.
Car body components which contribute to the safety of motor vehicles, particularly those components of the safety cage structure, are usually provided by form components with material enhancement such as by hardening in an associated apparatus, with the material characteristics being distributed in a uniform manner over the length of the longitudinal profile.
U.S. Pat. No. 5,192,376 issued on Mar. 9, 1993 discloses a reinforcing pipe for car bodies, comprised of a steel alloy, for the making of which steel is rolled, and the steel blank is rolled to shape, in the hot-rolled condition, at a temperature of 600 degrees Celsius or greater. Subsequently, the pipe produced in this manner is subjected to a quench hardening so as to increase its strength.
In consideration of the reasons concerning crashes, it can be of advantage that in the case of the so-called B-columns of the passenger cell of a motor vehicle, when this comprises regions with varying material properties such as strength and with varying elongation properties. A customary embodiment for this is a B-column comprising a two-component pressed part composite, with a lower pressed component of soft steel and a high-strength upper pressed component, whereby the two pressed components are joined by known joining techniques. This approach leads to an increased productive effort and to a greater weight of the B-column. Furthermore, in the event of a crash, the zone of the joint at times provides the weak link.
Federal Republic of Germany Patent No. 197 43 802, issued on Sep. 14, 2000, corresponding to U.S. Pat. No. 5,972,134, issued on Oct. 26, 1999, discloses a method to achieve a plastic stiffness behavior which is non-uniform or differentiated in pressed form parts which comprise enhanced material by being treated in associated apparatus.
Based on this prior art, it is thus an object of the invention to provide a B-column for a motor vehicle which is improved with respect to its crash behavior, the production cost, as well as in terms of a reduction in weight.
The solution of this object, in accordance with the invention, is achieved in a B-column comprising a longitudinal steel profile which has a first longitudinal portion comprised of a substantially predominantly martensitic material structure and a strength of more than 1400 N/mm2 and it, furthermore, comprises a second longitudinal portion with a greater ductivity or ductility and with a substantially predominantly ferritic-perlitic material structure and a strength of below 850 N/mm2.
The embodiment of the material of construction of the B-column is a steel which can be hardened. Particularly, there is of advantage a steel alloy comprised of, in weight percent:
balance iron, and including incidental impurities due to smelting.
The B-column is manufactured in a warm form process, starting with a form blank or a pre-formed longitudinal profile which is subjected in a furnace to a treatment yielding an austenitic material structure and, subsequently, it is re-formed/hardened in a cooled tool or equipment. Large areas of the blank or, respectively, the pre-formed longitudinal profile can be isolated or protected in the furnace against temperature effects. Thus, these areas are not subjected to a significant heating, such that the temperature increase overall in these areas is markedly below the temperature causing an austenitic material structure. It follows that there can be achieved, in the cooled tool, an absence of martensitic materials structure with the high strength.
In the region of the foot of the B-column there is essentially then predominant the original ferritic-perlitic structure of the starting materialxe2x80x94with excellent ductility (strength approximately 500 N/mm2, an elongation of approximately 20%). In the remainder of the component structure, there is present a substantially predominantly martensitic material structure with a tensile strength Rm of greater than 1400 N/mm2. 
It is also possible to first fully treat the longitudinal profile to attain an austenitic material structure and during transport into the hardening equipment, to bring the second longitudinal portion, that is, the subsequent foot of the column, by way of controlled, not too rough, cooling, for example, by blowing to a temperature markedly below the temperature which effects an austenitic material structure. The conditions such as, for example, residence time in the hardening equipment do then not produce a pure martensitic material structure, but a mixed structure with marked ferritic/bainitic components, which exhibits ductile properties.
A change from portions with high strength to ductile portions is possible with a plurality of frequencies.
The B-column in accordance with the invention exhibits an optimal crash behavior by the controlled pairing of strength and ductility. Furthermore, the invention reduces the number of components, as well as providing a reduction in weight, since additional joint zones between the otherwise separately produced structural components are dispensed with. Overall, this also yields a reduction in manufacturing cost.
In accordance with another feature of the present invention, the elongation at rupture A in the lower longitudinal portion is below 25%, preferably between 15% and 22%. Thus, the second longitudinal portion shows a markedly improved plastic stiffness behavior. The B-column is tuned in aimed manner to the requirements of a crash situation. In the first, upper, longitudinal portion it exhibits a high strength, whereas in the region of the second longitudinal portion (in the foot of the column) it exhibits high ductility.
In an advantageous embodiment of the present invention, the lower longitudinal portion is configured to be wider than the upper longitudinal portion. By way of this measure, as well as the feature that the longitudinal profilexe2x80x94preferably at the upper longitudinal portionxe2x80x94comprises a curved zone that is curved in spatial manner, the advantageous deformation characteristics of the longitudinal profile are enhanced in a crash event.